It is sometimes desirable to change the function of an electrical circuit, such as during the development stage of a new electronic device or during the manufacturing stage to implement a product change. Preferably, such a change can be implemented without adding to or deleting from the components which comprise the electrical circuit, including the traces or wires on any printed circuit board.
Frequently, such changes are implemented via software or firmware changes to programmable integrated circuit chips of the electrical circuit. One example of a programmable integrated circuit chip is the Maxim 214 IC made by Maxim Integrated Products of Sunnyvale California. This chip is designed to operate in conjunction with a Universal synchronous/Asynchronous Receiver/Transmitter (UART) to provide the various voltage levels required for two different communications interfaces. In one state, the Maxim 214 IC provides a serial port suitable for a Digital Communication Equipment (DCE) device. In another state, the Maxim 214 IC provides a serial port suitable for a Digital Terminal Equipment (DTE) device. The configuration state of the Maxim 214 IC is changed via external programmable control. That is, the Maxim 214 IC chip is provided with a control signal from a source external to the Maxim 214 IC chip which causes the Maxim 214 IC chip to change configuration states. To switch from DCE to DTE, or vice versa, no changes are required to the wiring or physical layout of the electrical circuit. In other words, to the human eye, there is no change to the electrical circuit.
However, programmable chips, such as the Maxim 214 IC, are typically more expensive than fixed function integrated circuit chips and require additional effort to program properly. In addition, it is not always foreseeable that such changes will be needed and designers therefore may not include such programmable chips in their designs. Accordingly, there remains a need for electrical circuits that allow for additional methods of changing the function of the electrical circuit.